1. Field of the Invention
The present invention relates to a step up/down converter for converting an input DC voltage from a power source into a predetermined DC voltage using an inductor, and supplies the predetermined DC voltage to a load, and particularly relates to a step up/down converter for controlling at a predetermined DC voltage based on an inductor current flowing through an inductor.
2. Description of the Related Art
In a step up/down converter, which converts an input DC voltage from a power source into a predetermined DC voltage by employing an inductor, and supplies it to a load, a configuration employing a H bridge circuit, in which inductor current flowing through an inductor is controlled by using four switches, is known (for example, see Japanese Laid-Open Patent Application Publication No. 2005-237052).
FIG. 6 is a circuit diagram, illustrating a schematic configuration of a conventional step up/down converter. As shown in FIG. 6, a step up/down converter comprises a DC power supply 105, an inductor 106, first to fourth switches 101 to 104, two of which are connected to each of both ends of the inductor 106, an output capacitor 107 for applying output voltage Vo to a load 108, a first current detecting resistor 91 for detecting electric current flowed to the third switch 103, a second current detecting resistor 92 to detect electric current flowed to the second switch 102, and a control circuit 110 for switching ON/OFF of the respective switches 101 to 104 based on electric current detected by the first and the second current detecting resistors 91 and 92.
In such a step up converter, the control circuit 110 provides control so as to achieve alternative (complementary) switching ON between the first switch 101 and the second switch 102 and to achieve the alternative (complementary) switching ON between the third switch 103 and the fourth switch 104. When the first switch 101 and the third switch 103 are switched ON and the second switch 102 and the fourth switch 104 are switched OFF (first state), an input voltage Vi of the DC power supply 105 is applied to the inductor 106, and energy becomes to be accumulated in the inductor 106 as the increase in the inductor current IL. Thereafter, the first switch 101 and the third switch 103 are switched OFF and the second switch 102 and the fourth switch 104 are switched ON (second state) to cause the output capacitor 107 to be charged with the energy accumulated in the inductor 106 through the second switch 102 and the fourth switch 104. The output capacitor 107 is charged with electric charge to increase the output voltage Vo applied to the load 108.
FIG. 7 is a graph, showing a signal waveform in each unit of the step up/down converter as shown in FIG. 6. At the time of the first state, the increasing inductor current IL is detected at the first electric current detecting resistor 91. In the step up/down converter shown in FIG. 6, when the detected voltage Vs10 based on the inductor current IL detected at the first electric current detecting resistor 91 reaches the upper limit current command value (voltage) V10 that is previously defined, the control circuit 110 switches the respective switches 101 to 104 to the second state. At the time of the second state, the decreasing inductor current IL is detected at the second electric current detecting resistor 92. In the step up/down converter shown in FIG. 6, when the detected voltage Vs20 based on inductor current IL detected at the second current detecting resistor 92 is reached to the previously defined lower limit current command value (voltage) V20, the control circuit 110 switches the respective switches 101 to 104 to the first state. As described above, the drive control of the respective switches 101 to 104 are achieved based on the inductor current IL detected by the current detecting resistors 91 and 92 corresponding to the respective states. Therefore, the upper limit current command value V10 and the lower limit current command value V20 are adjusted to control the output voltage Vo and the output current Io supplied to the load 108.    [Patent Literature 1] Japanese Laid-Open Patent Application Publication No. 2005-237052